108 NEKVOUS SYSTEM. 



the surrounding moisture, which, in his own experiments, 

 was carefully removed. 1 Longet has noted that pain is pro- 

 duced by the passage of a transverse current through a sen- 

 sitive trunk, and that the pain does not seem to be increased 

 when the poles are separated and the current thus is sent 

 through a portion of the length of the nerve. 2 



All who have experimented upon the action of galvanism 

 upon the mixed nerves have noted the fact alluded to above, 

 that the phenomena of contraction are manifested only on 

 closing or breaking the circuit. Take, for example, a frog's 

 leg prepared with the nerve attached ; place one pole of a 

 feeble galvanic apparatus on the nerve and then make the 

 connection, including a portion of the nerve in the circuit, 

 and usually, a contraction of the muscles will occur when the 

 circuit is closed, the limb will be quiet during the passage of 

 the current, and another contraction will take place when 

 the circuit is broken. "When the parts are freshly prepared, 

 the contractions take place as described,, whatever be the 

 direction of the current. 3 After a time, however, the ner- 

 vous irritability becomes somewhat enfeebled, and then it is 

 observed that the contraction occurs in some instances when 

 the circuit is closed, and in others when the circuit is broken. 

 The differences in the time of appearance of these phenom- 

 ena have been found to depend upon the direction of the 

 current, and may be formularized as follows : 



If the sciatic nerve attached to the leg of a frog, prepared 



1 CHAUVEAU, Effets physiologiques de Velectridte. Journal de la physiologic, 

 Paris, 1860, tome Hi., p. 298. 



2 LONGET, loc. cit., p. 201. 



3 A form of galvanic apparatus which we have long used and found very 

 convenient for these experiments is essentially the one described by Bernard 

 (Systeme nerveux, Paris, 1858, tome i., p. 144). It consists simply of alternate 

 copper and zinc wires wound around a piece of wood bent in the form of a 

 horseshoe and terminating in two platinum points representing the positive and 

 negative poles. This forms a sort of electric forceps, about eight inches long, 

 which, when moistened with water slightly acidulated with acetic acid, will give 

 a current of about the strength required for most of the experiments detailed 

 above. 



